This invention relates to the investigation of earth formations and, more particularly, to an apparatus and method for determining the bound-water-filled porosity of formations surrounding a borehole.
Modern well logging technology has advanced to a point where a number of subsurface parameters, for example porosity and lithology, can often be determined with reasonable accuracy. However, a reliable technique for determining the permeability of formations (i.e., a measure of the ease with which fluid can flow through a pore system), has not been forthcoming. Resistivity gradients have been used to estimate the order of magnitude of formation permeability, but this technique is found useful only in certain types of formations. It has been suggested that a measurement of the amount of "free fluid" in shaly formations would be a good permeability indicator. A known technique for measuring "free fluid" is the nuclear magnetic resonance tool, but attainable signal-to-noise ratios tend to be a limiting factor of performance for this tool.
In the copending U.S. patent application Ser. No. 674,791, now U.S. Pat. No. 4,063,151, of R. Rau and J. Suau, assigned to the same assignee as the present application, there is disclosed a technique for determining the amount of bound water in formations surrounding a borehole by measuring the dielectric constant of the formations at two different microwave frequencies. The difference between the measurements taken at the two frequencies is utilized to determine the amount of bound water in the formations. In another copending U.S. patent application Ser. No. 674,792, now U.S. Pat. No. 4,077,003, of R. Rau, also assigned to the present assignee, determinations of dielectric loss factors are taken at two different frequencies and are utilized to obtain information about the amount of bound water in the formations.
It is one object of the present invention to determine, without the need for dual frequency measurements, the bound-water-filled porosity of formations surrounding a borehole.